Structural repair having optical witness and method of monitoring repair performance

ABSTRACT

A structural repair includes a tell-tale optical witness that allows the health of the repair to be visually monitored. The optical witness includes a stress sensitive fluorescent dye that shows changes in local strain/stress patterns when the repair is subjected to electromagnetic energy of a particular wavelength. The dyes fluoresce more or less strongly as a function of the local stress/strain.

This application is a divisional application of U.S. patent applicationSer. No. 13/310,539, filed Dec. 2, 2011 which claims priority fromprovisional application No. 61/482,737.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Provisional U.S. PatentApplication No. 61/482,737 filed May 5, 2011, which is incorporated byreference herein in its entirety. This application is also related toco-pending U.S. patent application Ser. No. 13/310,333, filed on evendate herewith, which is incorporated by reference herein in itsentirety.

BACKGROUND INFORMATION Field

The present disclosure relates generally to manufacturing and servicingvehicles, especially aircraft. More particularly, the present disclosurerelates to a method of repairing or reworking inconsistencies incomponents of the aircraft that allow the performance of the repair tobe monitored using optical inspection techniques.

Background

Monitoring the health, state, and/or quality of on-aircraft bondedstructural repairs, such as doubler repairs on composites or metals, canbe expensive and challenging using traditional NDE (non-destructiveevaluation) equipment. The use of a trained NDE technician to inspect arepair for signs of early degradation can be time consuming and mayrequire the aircraft to be taken out of service while the inspection isperformed. Also, bond testers and ultrasonic test equipment typicallyused to perform the inspection maybe too costly for smaller airlines.Current techniques for structural health monitoring of repairs requireequipment that flies with the airplane or connects into circuitryattached to sensors that are pulsed to check the repair with ultrasonicstructural waves. These systems are costly and can add undesired weightto the aircraft.

Accordingly, there is a need for a simple and rapid method of detectinginitial signs of degradation of a repair, such as a loss of adhesion ora change in its strain pattern. There is also a need for a repair thatincorporates a tell-tale feature allowing optical observation ofdegradation or other changes during routine, periodic service checks.

SUMMARY

An optical witness or ‘tell-tale’ feature is incorporated into astructural repair that allows the performance of the repair to bequickly and easy monitored. Changes in the repair, including earlypartial failure or degradation of a repair can be visually identified sothe repair can be more frequently monitored, repaired, or replaced asneeded. In one embodiment, stress sensitive fluorescent dyes areincorporated into the pigment of an appliqué placed over the repair areaor in the resin of the surface or overlay ply of the repair. The opticalbehavior of the fluorescent dye changes as a function of a stress in therepair.

The stress sensitive fluorescent dye, which may be referred to asmechanochromatic dyes, may be designed to show changes in localstrain/stress patterns, so that when they are subjected toelectromagnetic energy of a particular wavelength, such as UV, IR, orvisual light, they fluoresce more (or less) strongly. When the bond in arepair begins to degrade, the adhesive disbonds from the repairedstructure causing the local strain within the patch and/or surroundingstructure to change. The strain may be relatively low over the disbondedarea, but may rise in other areas as the patch and/or structure attemptsto carry the load. These changes will may seen by an inspectiontechnician performing a quick visual check of the patch using anappropriate light source to fluoresce the dye in the appliqué or surfaceply. If the fluorescence over an area of the patch becomes non-uniformor different from its baseline, this may be taken as an indication thatthe patch is beginning to degrade in that area, and should be checkedwith an NDE instrument or regularly monitored until it can be checked,repaired, or replaced.

The cost of incorporating a witness dye into an appliqué or in the resinof a surface ply is relatively low. The inspection/monitoring methoddoes not require a highly trained NDE technician or expensive NDEequipment. The inspection method and optical witness may reduce oreliminate the need for complicated and expensive structural healthmonitoring (SHM) equipment or on-board sensors. The mechanochromatic dyemay be used at a relatively low level in order to avoid significantlyincreasing the weight of a resin in an overlay ply. In some embodiments,when the dye is incorporated into an appliqué, the appliqué may alsofunction as lightning strike protection, and may be selected to providea color that matches the structure around it.

According to one disclosed embodiment, a structural repair comprises apatch adapted to be adhesively bonded to a structure requiring repair,and a layer of material covering the patch for visually indicatingchanges in the repair. The layer of material may include a composite plyoverlying and cocured with the patch, wherein the composite ply containsa mechanochromatic dye. The mechanochromatic dye fluoresces whensubjected to electromagnetic energy in accordance with localizedstresses in the repair. In another embodiment, the layer of material mayinclude an appliqué adhered to the structure and covering the patch,wherein the appliqué contains a mechanochromatic dye having an opticalcharacteristic that changes in accordance with localized stresses in therepair.

According to another disclosed embodiment, a method is provided ofmonitoring changes in a patch bonded to a structure. The methodcomprises applying a layer of material over the patch having an opticalbehavior that varies in response to changes in stress in the patch, andperiodically checking the layer of material for changes in the opticalbehavior of the layer of material. Applying the layer of material mayinclude adhering an appliqué to the structure overlying the patch, whilein another embodiment, applying the layer of material includes placing acomposite ply over the patch, and cocuring the composite ply and thepatch. Periodically checking the layer of material includes subjectingthe layer of material to electromagnetic energy of a preselectedwavelength, and recording the optical behavior of the layer of material.Recording the optical behavior of the layer of material includescollecting photoluminescent quantum yield and fluorescence emissionspectra from the layer of material. Periodically checking the layer ofmaterial may further include marking an area of the structure containingthe patch when the optical behavior indicates a change in the stress inthe patch, and performing further non-destructive evaluation of thepatch.

According to still another embodiment, a method is provided of repairingan area of an aircraft component. The method comprises placing a patchover the area; bonding the patch to the aircraft component; placing alayer of material over the patch having an optical behavior that variesin response to changes in stress in the patch; recording a baselineimage representing the optical behavior of the layer of material at atime that the patch is bonded to the aircraft component; andperiodically checking performance of the patch by recording subsequentimages representing the optical behavior of the layer of material, andcomparing the subsequent images to the baseline image. Placing the layerof material may include adhering an appliqué to the aircraft componentoverlying the patch or placing a composite ply over the patch, andcocuring the composite ply and the patch. Recording the baseline imageand recording the subsequent images each includes subjecting the layerof material to electromagnetic energy of a preselected wavelength, andcollecting photoluminescent quantum yield and fluorescence emissionspectra from the layer of material. Periodically checking theperformance of the patch may further include marking an area of theaircraft component containing the patch when comparison of thesubsequent images to the baseline image indicates a change in the stressin the patch, and performing further non-destructive evaluation of thepatch. Comparing the subsequent images to the baseline image isperformed using a data processing system.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the advantageousembodiments are set forth in the appended claims. The advantageousembodiments, however, as well as a preferred mode of use, furtherobjectives and advantages thereof, will best be understood by referenceto the following detailed description of an advantageous embodiment ofthe present disclosure when read in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is aircraft manufacturing and service method in which anadvantageous embodiment may be implemented;

FIG. 2 is aircraft in accordance with an advantageous embodiment;

FIG. 3 is an illustration of an aircraft in accordance with anadvantageous embodiment;

FIG. 4 is an illustration of a network of data processing systems inaccordance with an advantageous embodiment;

FIG. 5 is an illustration of a data processing system in accordance withan advantageous embodiment;

FIG. 6 is a system for determining stress inconsistencies in an aircraftcoating in accordance with an advantageous embodiment;

FIG. 7 is a cross section of a composite component in accordance with anadvantageous embodiment;

FIG. 8 is a cross section of a composite component havinginconsistencies therein;

FIGS. 9A-9F are perspective views of a portion of an aircraft skinshowing the successive steps of a method of repairing inconsistencies inthe skin and inspecting the repair.

FIG. 9G is a cross sectional view of an aircraft skin, showing anoptical witness appliqué in the process of being placed over a repairpatch in the skin.

FIG. 9H is a cross sectional view similar to FIG. 9G but showing theappliqué having been applied flush on the skin covering the repairpatch.

FIG. 9I is a flowchart of a method of monitoring changes in a compositepatch bonded to a structure.

FIG. 9J is a flowchart of a method of repairing an area of an aircraftcomponent.

FIG. 10 is flowchart for applying a stress sensitive fluorescent coatingin accordance with an advantageous embodiment;

FIG. 11 is a flowchart of a process for determining stressinconsistencies in accordance with an advantageous embodiment;

FIG. 12 is a flowchart for servicing a component having an inconsistentstress profile in accordance with an advantageous embodiment;

FIG. 13 shows the Differential Scanning calorimetry scans resulting frommeasurements of uncured and cured epoxy films in accordance with anadvantageous embodiment;

FIG. 14 shows the Differential Scanning calorimetry scans resulting frommeasurements of uncured and cured polyurethane coatings in accordancewith an advantageous embodiment;

FIG. 15 shows representative plots of the Differential Scanningcalorimetry scans taken for epoxy and polyurethane coatings within theincorporated stilbene-type fluorescent dyes in accordance with anadvantageous embodiment;

FIG. 16 shows Glass transition temperature measurements of epoxy andpolyurethane coatings with and without the incorporated stilbene-typefluorescent dyes in accordance with an advantageous embodiment;

FIG. 17 shows the absorbance spectra for the tetra-butyl dimethyl silanefunctionalized stilbene dyes at various concentrations in the “thinner”precursor of the polyurethane coating in accordance with an advantageousembodiment;

FIG. 18 shows the absorbance spectra for the hydroxyl functionalizedstilbene dyes at various concentrations in the thinner precursor of thepolyurethane coating in accordance with an advantageous embodiment;

FIG. 19 shows the absorbance of the tert-butyl dimethyl silanefunctionalized stilbene dyes in solid polyurethane films on glasssubstrates in accordance with an advantageous embodiment.

FIG. 20 shows typical stress-strain curves for the PET-epoxy andPET-polyurethane bilayers in accordance with an advantageous embodiment;

FIG. 21 shows images from a tensile stress test of hydroxylfunctionalized stilbene dyes in an epoxy film in accordance with anadvantageous embodiment; and

FIG. 22 shows images from a tensile stress test of hydroxylfunctionalized stilbene dyes in a polyurethane film in accordance withan advantageous embodiment.

DETAILED DESCRIPTION

The disclosed embodiments relate to an optical witness or ‘tell-tale’feature incorporated into a structural repair that allows theperformance of the repair to be quickly and easy monitored. Changes inthe repair, including early partial failure or degradation of a repaircan be visually identified so the repair can be more frequentlymonitored, repaired, or replaced as needed. The optical witness maycomprise stress sensitive fluorescent dyes incorporated into the pigmentof an appliqué placed over the repair area or into the resin of thesurface or overlay ply of the repair. The optical behavior of thefluorescent dye changes as a function of a stress in the repair, therebyproviding a visual indication of the changes in the repair.

Referring to the drawings, embodiments of the disclosure may bedescribed in the context of aircraft manufacturing and service method100 as shown in FIG. 1 and aircraft 200 as shown in FIG. 2. Turningfirst to FIG. 1, during pre-production, aircraft manufacturing andservice method 100 may include specification and design 102 of aircraft200 in FIG. 2 and material procurement 104. During production, componentand subassembly manufacturing 106 and system integration 108 of aircraft200 in FIG. 2 takes place. Thereafter, aircraft 200 in FIG. 2 may gothrough certification and delivery 110 in order to be placed in service112. While in service 112 by a customer, aircraft 200 in FIG. 2 isscheduled for routine maintenance and service 114, which may includemodification, reconfiguration, refurbishment, and other maintenance orservice, including the inspection and repair of components andsubassemblies.

Each of the processes of aircraft manufacturing and service method 100may be performed or carried out by a system integrator, a third party,and/or an operator. In these examples, the operator may be a customer.For the purposes of this description, a system integrator may include,without limitation, any number of aircraft manufacturers andmajor-system subcontractors; a third party may include, withoutlimitation, any number of vendors, subcontractors, and suppliers; and anoperator may be an airline, a leasing company, a military entity, aservice organization, and so on.

With reference now to FIG. 2, an illustration of an aircraft is depictedin which an advantageous embodiment may be implemented. In this example,aircraft 200 is produced by aircraft manufacturing and service method100 in FIG. 1 and may include an airframe 202 with the plurality ofsystems 204 and the interior 206. Examples of the systems 204 includeone or more of a propulsion system 208, an electrical system 210, ahydraulic system 212, and an environmental system 214. Any number ofother systems may be included. Although an aerospace example is shown,different advantageous embodiments may be applied to other industries,such as the automotive industry.

Structural repairs and repair methods embodied herein may be employedduring at least one of the stages of the aircraft manufacturing andservice method 100 in FIG. 1. As used herein, the phrase “at least oneof”, when used with a list of items, means that different combinationsof one or more of the listed items may be used and only one of each itemin the list may be needed. For example, “at least one of item A, item B,and item C” may include, for example, without limitation, item A or itemA and item B. This example also may include item A, item B, and item Cor item B and item C.

In one illustrative example, components or subassemblies produced in thecomponent and subassembly manufacturing 106 in FIG. 1 may be fabricatedor manufactured in a manner similar to components or subassembliesproduced while the aircraft 200 is in service 112 in FIG. 1. As yetanother example, a number of apparatus embodiments, method embodiments,or a combination thereof may be utilized during production stages, suchas the component and subassembly manufacturing 106 and the systemintegration 108 in FIG. 1. A number, when referring to items, means oneor more items. For example, a number of apparatus embodiments is one ormore apparatus embodiments. The use of a number of the differentadvantageous embodiments may substantially expedite the assembly ofand/or reduce the cost of aircraft 200. A number of apparatusembodiments, method embodiments, or a combination thereof may beutilized while the aircraft 200 is in service 112 and/or duringmaintenance and service 114 in FIG. 1. For example, the disclosedembodiments may be employed to repair components and subassemblies ofthe aircraft 200 during maintenance and service 114. Also, methodembodiments disclosed herein may be employed during maintenance andservice 114 to monitor the performance of previously made structuralrepairs.

With reference now to FIG. 3, an illustration of an aircraft 300 isdepicted in which an advantageous embodiment may be implemented.Aircraft 300 is a typical example of the aircraft 200 shown in FIG. 2,in which repairs utilizing stress sensitive fluorescent coatings may beimplemented. In this illustrative example, the aircraft 300 has wings302 and 304 attached to a body 306. The aircraft 300 includes a wingmounted engine 308, a wing mounted engine 310, and a tail 312. Each ofthe wings 302, 304, the body 30, the wing mounted engine 308, the wingmounted engine 310, and the tail 312 may include components, such as anouter skin which may include composite patch repairs incorporatingstress sensitive fluorescent coatings that allow the performance of therepairs to be monitored.

Monitoring changes in composite patches in accordance with the disclosedembodiments may be employed to develop inspection data used to manage amaintenance program for an aircraft or a fleet of aircraft. FIG. 4 is anillustration of a network data processing system 400 in which theadvantageous embodiments may be implemented as part of an aircraftmaintenance program. The network data processing system 400 comprises anetwork 402 which is the medium used to provide communications linksbetween various devices and computers connected together within networkdata processing system 400. The network 402 may include connections,such as wire, wireless communication links, or fiber optic cables. Inthe illustrated example, servers 404, 406 connect to the network 402along with a storage unit 408, and clients 410, 412, and 414. Theseclients 410, 412, and 414 may be, for example, personal computers ornetwork computers. In the depicted example, the server 404 providesdata, such as boot files, operating system images, and applications tothe clients 410, 412, and 414. The clients 410, 412, and 414 are clientsto the server 404 in this example. One or more aircrafts 416 are alsoclients that may exchange information with clients 410, 412, and 414.

In the illustrated depicted example, a photoluminescent device 418connects to one or more of the servers 404, 406, the clients 410, 412,414. The photoluminescent device 418 functions to collectphotoluminescent quantum yield (PLQY) and fluorescence emission spectraindicative of the performance of a composite structural repair patch.Photoluminescent device 418 can be for example, a Hamamatsu Absolute PLQuantum Yield Measurement System available from Hamamatsu K.K, USlocation Bridgewater, N.J. Photoluminescent device 418 can obtainfluorescence profiles of composite structural repair patches and storethose fluorescence profiles on one or more of the server 404, the server406, the client 410, the client 412, and the client 414.

FIG. 5 illustrates a data processing system 500 that may be used toimplement the servers and clients shown in FIG. 4, and is typical of asystem that may be found on the aircraft 416 in FIG. 4. The dataprocessing system 500 broadly comprises a communications framework 502,processor unit 504, storage devices 506, 508, communications unit 510,input/output unit 512 and a display 514. As depicted, data processingsystem 500 includes communications framework 502, which providescommunications between processor unit 504, storage devices 506, 508,communications unit 510, input/output unit 512, and display 514. In somecases, communications framework 502 may be implemented as a bus system.The processor unit 504 executes instructions for software that may beloaded into the storage devices 506, 508. The communications unit 510may provide communications through the use of either or both physicaland wireless communications links. Input/output unit 512 allows forinput and output of data with other devices that may be connected to thedata processing system 500. The data processing system may employ onemore computer programs 522 on computer readable media 520 which mayinclude program code 518, computer readable storage media 524 andcomputer readable signal media 526.

As previously mentioned, the disclosed embodiments provide a structuralrepair whose condition or performance can be monitored by periodicvisual inspection during routine service checks of the aircraft. Changesin the repair that are visually identified during these routine servicechecks may indicate that a repair patch should be monitored morefrequently, repaired further or replaced. As will be discussed below inmore detail, repairs made according to the disclosed method employstress sensitive fluorescent dyes that are incorporated into the pigmentof an appliqué placed over the repair area or into the resin of thesurface or overlay ply of the repair. The optical behavior of thefluorescent dye changes as a function of a stress in the repair, therebyproviding a visual indication of the changes in the repair.

Highly efficient, aggregation-sensitive dyes with intrinsic dipolemoments are selected and functionalized with end groups to eitherpromote or prevent combination with coating polymer networks. Whenstress is applied to the coating or layer of material over a compositerepair patch, the positions of the dye molecules will shift as thepolymer network displaces. The applied stress will change the dyes'aggregation behavior, and change their fluorescence behavior as aresult.

The viscoelastic nature of polymer coatings and layers, and thecomplexity of molecular interactions make predicting the manner in whicha dye will respond challenging. However, according to the solutionprovided by the disclosed embodiments, the manner in which the dye'sfluorescence behavior changes is not important. Moreover, the discloseddyes used to monitor changes in a structural repair are not dependent onthe presence of a particular type of stress—the dye is sensitive to bothtensile and compressive stresses in a repair patch caused by patchdegradation, disbonding or other factors. Changes in fluorescentwavelength emission, either toward monomer-like behavior or dimer-likebehavior, or changes in emission intensity due to quenching oraggregation-induced emission, are all detectable. By comparing aninitial stress profile of aircraft coatings or layers incorporating thedyes to subsequent stress profiles, stress changes in a structuralrepair patch can be detected.

FIG. 6 illustrates a system used to carry out a method for monitoringchanges in a composite patch used to repair a component 612 of the anaircraft 610 which may be similar to that previously described inconnection with FIG. 3. Component 612 may comprise, for example, acomposite outer skin. A stress sensitive fluorescent coating 614 isplaced over areas of the component 612 that contains structural repairs.As will be discussed later in more detail, the repair may include acomposite laminate patch, and the coating 614 may be in the form of anappliqué or a layer that overlies the composite patch. The condition ofthe repair may be monitored by determining stress inconsistencies in thecoating 614 covering the repair. The stress inconsistencies may resultfrom, for example, degradation, delamination or disbonding of the repairpatch from the aircraft component 610.

The stress sensitive fluorescent coating 614 includes fluorescent dyemolecules 616 whose fluorescent behaviors change in response to externalstress or deformation stimuli. The fluorescent dye molecules 616 displaya behavior that depends on their concentration within the localenvironment. If two dye molecules are in very close proximity to oneanother, they may share the energy of an absorbed photon between them bymerging their electron density to form a dimer complex. The dimercomplex absorbs and emits photons at differing wavelengths and withdifferent efficiency than the single molecule or monomer. Thisphenomenon may also be referred to as aggregation.

The induced fluorescence of the fluorescent dye molecules 616 changeswith deformation of the component 612 in the area of the repair. As thelocal environment of the fluorescent dye molecules 616 is deformed, theproximity of dye molecules to one another is changed, either increasedor decreased depending on, for example, the molecular mobility of thefluorescent dye molecules 616. The probability of the fluorescent dyemolecules 616 to form aggregates is then also changed, and as a resultthe fluorescence behavior of the fluorescent dye molecules 616 ischanged as well.

In an advantageous embodiment, the fluorescent dye molecules 616 withintrinsic dipole moments are selected and incorporated into the stresssensitive fluorescent coating 614. When stress is applied to the stresssensitive fluorescent coating 614, which may be caused by degradation,delamination or disbonding of the repair patch beneath the coating 614,the positions of the fluorescent dye molecules 616 shift as the polymernetwork displaces. This shift changes the aggregation behavior of thefluorescent dye molecules 616, and therefore also the fluorescencebehavior as a result.

In one advantageous embodiment, the fluorescent dye molecules 616 arebased on a modified stilbene-type fluorescent molecule customized withdiffering end groups designed to control their solubility andinteraction with the polymer coating components of the stress sensitivefluorescent coating 614. The modified stilbene-type fluorescent moleculeexhibits a large amount of conjugation that allows its electron densityto move both within the molecule, for monomer-type excitation, andout-of-plane when in proximity with another stilbene, for dimerexcitation The data processing system 620 can be, for example, one ormore of the server 404, the server 406, the client 410, the client 412,and the client 414 of FIG. 4.

Referring now to FIG. 7, a cross section of a composite component isshown according to an advantageous embodiment. The composite component700 can be a component such as an aircraft outer skin. The compositecomponent 700 may include layers 710-718 which form laminated plies.Each of the layers 710-718 may comprise a fibrous reinforcement that hasbeen impregnated with a polymeric resin. Layers 710-718 are laminatedtogether to form a substantially consolidated structure.

In one embodiment, a stress sensitive fluorescent coating 720 is appliedto the composite component 700, and may comprise the stress sensitivefluorescent coating 614 shown in FIG. 6. The stress sensitive coating720 may comprise a topcoat of paint or other material and/or anunderlying primer coat that incorporates the fluorescent dye molecules722. In some embodiments, the topcoat to which the stress sensitivecoating 720 is applied may be a clearcoat substantially devoid ofpigmentation. In still other embodiments, the fluorescent dye molecules722 may be incorporated into the first layer (ply) 710 of resin duringfabrication of the composite component 700. In still other embodiments,as will be discussed in more detail below, the stress sensitive coating720 may comprise an appliqué or a layer of composite material covering arepair in the composite component 700.

The stress sensitive fluorescent coating 720 exhibits a stress profilebased on the local environment of the fluorescent dye molecules 722. Thefluorescent dye molecules 722 that are in a particular proximity toothers of the fluorescent dye molecules 722 due to stress of the stresssensitive fluorescent coating 720 will exhibit fluorescence that isdifferent than the fluorescent dye molecules 722 that are in a differentproximity to others of the fluorescent dye molecules 722. In thoseapplications where the fluorescent dye molecules 722 are incorporatedinto the first layer 710 of the component 700, the response of thecoating 720 may be obscured by any overlying topcoat or primer coat thatmay be applied to the component 700. However, prior to the applicationof any topcoat and/or primer coat, such as during an intermediate stageof manufacturing or before the component 700 is placed in service, thereaction of the fluorescent dye molecules 722 in the first layer 710 mayreveal damage or other phenomena that cause stress concentrations on thecomponent 700.

Referring now to FIG. 8, a cross section of a composite component 800that has experienced an event causing an inconsistency that may beinvisible or barely visible to a visual inspection, is shown accordingto an advantageous embodiment. The inconsistency may be in an area ofthe component 800 that contains a repair. The composite component 800includes a plurality of laminated layers 810-818 of fiberousreinforcement impregnated with a polymeric resin. The stress sensitivefluorescent coating 820 may be applied to the surface of the compositecomponent 800. The stress sensitive fluorescent coating 820 is thestress sensitive fluorescent coating 614 of FIG. 6. The compositecomponent 800 includes inconsistencies 830 that may be invisible orbarely visible to a visual inspection. The inconsistencies 830 mayinclude, for example, delaminations of one or more of the layers810-818, undesired conditions in resins of the composite component 800,and/or undesired conditions in fiber reinforcement of the compositecomponent 800. In the case of a repair made to the composite component800, the inconsistency 830 may comprise, for example, delaminationwithin a laminated composite repair patch, disbonding or degradation ofthe patch or other changes in the repair that may affect the performanceof the component 800.

Events resulting in the inconsistencies 830 cause changes in a stressprofile of the stress sensitive fluorescent coating 820. Changes in thestress profile may also be detected which are indicative of aninconsistency caused by any of a variety of events, including but notlimited to ply delamination within a repair patch, disbonding ordegradation of the patch or impact damage to the patch. The inducedfluorescence of the fluorescent dye molecules 616 therefore also changesstress profile of the stress sensitive fluorescent coating 820. As thelocal environment of the fluorescent dye molecules 822 is deformed, theproximity of dye molecules to one another is changed, either increasingor decreasing depending on, for example, the molecular mobility of thefluorescent dye molecules 616. The probability of the fluorescent dyemolecules 616 to form aggregates is then also changed, and as a result,the fluorescence behavior of the fluorescent dye molecules 616 ischanged as well.

The viscoelastic nature of the stress sensitive fluorescent coating 820and the complexity of molecular interactions of the fluorescent dyemolecules 822 make predicting the manner in which a dye will responddifficult. However, changes in fluorescent wavelength emission, eithertoward monomer-like behavior or dimer-like behavior, or changes inemission intensity due to quenching or aggregation-induced emission, areall detectable. By comparing an initial stress profile of the stresssensitive fluorescent coating 720 of FIG. 7 to the subsequent stresssensitive fluorescent coating 820, stress changes due to the presence ofinconsistencies in a component 800, including repairs made to thecomponent 800 can be determined.

Referring now to FIG. 9A, the disclosed stress sensitive fluorescentcoating 920 may be employed to monitor changes in a repair or reworkarea 960 within a component 900 which may comprise, for example andwithout limitation, a composite skin 900. The stress sensitivefluorescent coating 920 exhibits a stress profile based on the localenvironment of the fluorescent dye molecules 922. FIG. 9A illustrates across section of the skin 900 after having undergone a process to repairone or more inconsistencies caused for example, by an impact. The repairarea 960 comprises a scarf 940 in the skin 900 that is covered andfilled with an adhesively bonded repair patch 928. The repair patch 928includes laminated composite plies 950-958 which may be aligned with thelayers 910-918 of the skin 900. In this example, the stress sensitivefluorescent coating 920 is applied to the surface of the skin 900,overlying the repair patch 928. The stress sensitive fluorescent coating920 may be incorporated into a topcoat paint or primer that is appliedover the entire area of the skin 900, as during repainting of theaircraft 300 (FIG. 3), or only over a portion of the area of the skin900. The stress profile for the skin 900, including that of the repairarea 960, may be different respectively before and after changes thatmay occur in the repair area 960, including the repair patch 928. Thesechanges may represent inconsistencies in the repair area 960 resultingfrom one or more events, conditions or phenomena, including but notlimited to impact damage to the repair patch 928, or delaminationdisbonding or degradation of the repair patch 928. Therefore, afterperforming a repair or other rework operation, a new stress profile isobtained for the composite skin 900. The new stress profile can then beinput and stored in a data processing system, such as the dataprocessing system 620 of FIG. 6, for use in subsequent inspection andservicing of the aircraft 300, and monitoring of the condition of therepair patch 928.

FIGS. 9B-9F illustrate another method of repairing or reworking acomposite component using the disclosed stress sensitive fluorescentcoating 920. As shown in FIG. 9B, a composite component, which in theillustrated example comprises an aircraft skin 900, has inconsistencies926 such as, for example, impact damage that may or may not be visible.The inconsistencies 926 may be removed in a repair area 960 using ascarfing technique similar to that previously described in connectionwith FIG. 9A. Referring to FIG. 9C, a composite patch 928 is adhesivelybonded to the skin 900 in the repair area 960 containing theinconsistencies 926. The composite patch 928 may comprise, for example,multiple laminated plies of composite material such as fiber reinforcedresin. Next, as shown in FIG. 9D, an overlay layer 930 of material isplaced over the composite patch 928. The overlay layer 930 of materialcontains a mechanochomatic or stress sensitive fluorescent dye of thetype previously discussed in connection with FIGS. 7 and 8 whichincludes stress sensitive fluorescent dye molecules. Themechanochromatic dyes may be designed to show changes in localstrain/stress patterns, so that when they are subjected toelectromagnetic energy of a particular wavelength, such as UV, IR, orvisual light, they fluoresce more (or less) strongly as a function ofthe local stress/strain.

In one embodiment, the overlay layer 930 may comprise an overlay ply ofcomposite material such as fiber reinforced resin that is cocured withthe composite patch 928. In other embodiments, the overlay layer 930 ofmaterial may comprise an appliqué 952 (See FIGS. 9G and 9H) that ispressed into place on the skin 900 over the patch 928 in the repair area960. The appliqué 952 may comprise a polymeric film or other suitablematerial which contains the mechanochomatic dye and is adhered to theskin 900. The mechanochromatic dye may be used at a relatively low levelin order to avoid significantly increasing the weight of a resin in anoverlay ply. In some embodiments, when the dye is incorporated into anappliqué 952, the appliqué 952 may also function as lightning strikeprotection, and may be selected to provide a color that matches thestructure (e.g. skin 900) around it. The mechanochomatic dye may betailored to respond to specific type of repair degradations.

FIG. 9E illustrates the next step in the repair/rework method, in whichelectromagnetic radiation 932 of a suitable wavelength generated from aphotoluminescent device, which may comprise a suitable light source 938,is directed onto the repair patch 928. The wavelength of the light 932may be, for example, in the UV (ultraviolet) range, depending on theparticular mechanochomatic dye contained in the overlay layer 930 ofmaterial that overlies the composite patch 928. Illumination of thecomposite patch 928 with the radiation 932 produces a baseline strainimage or stress profile that essentially comprises a mapping of specificfluorescence of components of the mechanochomatic dye. This process ofproducing a strain image is essentially the same as that previouslydescribed in connection with FIG. 6 in which a photoluminescent device618 is used to collect Photoluminescent quantum yield (PLQY) andfluorescence emission spectra. In FIG. 6, photoluminescent device 618may generate electromagnetic radiation 622. The baseline strain imagemay be stored in the data processing system 400, 500 shown in FIGS. 4and 5 respectively, for future use in servicing and monitoring thehealth of the aircraft 300 shown in FIG. 3. During periodic checks tomonitor the performance of the repair patch 928, subsequent strainimages are recorded which are compared with the baseline strain imageusing the data processing system 400, 500.

The initial baseline strain image may directly reveal, for example,areas of the composite patch 928 that are inadequately bonded to theskin 900. When the bond in a repair begins to degrade, the adhesivedisbonds from the repaired structure, causing the local strain withinthe patch 928 and/or surrounding structure to change. The strain may berelatively low over the disbonded area, but may rise in other areas asthe patch 928 and/or structure attempts to carry the load. These changesmay seen by an inspection technician performing a quick visual check ofthe patch 928 using an appropriate light source to fluoresce the dye inthe appliqué or surface ply. If the fluorescence over an area of thepatch 928 becomes non-uniform or different from its baseline, this maybe taken as an indication that the patch 928 is beginning to degrade inthat area, and may be checked with an NDE instrument or regularlymonitored until it can be checked, repaired, or replaced. Any areas ofthe composite patch 928 revealed to possibly have inadequate bonding mayfurther evaluated using any of several known NDE (non-destructiveevaluation) techniques.

Referring now to FIG. 9F, the repair patch 928 may be quickly and easilyperiodically checked by service personnel for undesirable changes duringroutine servicing of the aircraft 300. A typical repair check comprisesdirecting UV light 932 onto the repair area 960 and comparing theresulting fluorescence with the stored baseline strain image, using thedata processing system 400, 500 (FIGS. 4 and 5). Areas 934 exhibiting anincrease in fluorescence may indicate an increase in the stress/strainas the load path through the repair area 960 shifts, while areas 936that exhibit a decrease in fluorescence may indicate a reduction of loadcarrying ability of the patch 928 at the edges of the patch 928. Itshould be note here that while the exemplary embodiments illustrate theuse of the repair and monitoring method in connection with the repair ofa composite structure, the embodiments may also be used in the repair ofother types of structures, such as without limitation, aluminum andtitanium structures employing bonded patches.

FIG. 9G illustrates an optical witness appliqué 952 being placed on thesurface 954 of a skin 900. The appliqué 952 may comprise, for example, aflexible sheet-like material that may be bonded to the skin surface 954using a suitable adhesive. Alternatively, the appliqué 952 may itselfcomprise a sheet of adhesive that incorporates the stress sensitivefluorescent dyes therein. FIG. 9H shows the appliqué 952 adhered andlying flush on the skin surface 954. The appliqué 952 has an area thatis at least sufficient to cover the entire area of the patch 928, butpreferably extends beyond the outer margins 956 of the patch 928.

FIG. 9I broadly illustrates the steps of a method of monitoring changesin a composite patch 928 bonded to a structure, such as an aircraft skin900, as discussed above. At step 970, a layer of material 930 is appliedover the patch 928. The layer of material 930 has an optical behaviorthat varies in response to changes in stress in the patch 928. At step972, the layer of material 930 is periodically checked for changes inits optical behavior. As previously mentioned, changes in the opticalbehavior of the layer of material 930 may indicate changes in thecondition or performance of the patch 928.

FIG. 9J broadly illustrates the steps of a method of repairing an area960 of an aircraft component 900, which may comprise an aircraft skin900. Beginning at 974, a patch 928 is placed over the repair area 960 onthe component 900, following which, at 976, the patch 928 is bonded tothe component 900. At step 978, a layer of material 930 is placed overthe patch 928. The layer of material 930 has an optical behavior thatvaries in response to changes in the stress in the patch 928. At 980, abaseline image of the optical behavior of the layer of material 930 isrecorded at the time that the patch 928 is bonded to the component 900.At step 982, the performance of the patch 928 is periodically checked byrecording subsequent images of the layer of material 930 and comparingthe subsequently recorded images with the baseline image.

Referring now to FIG. 10, a flowchart for a process 1000 for applying astress sensitive fluorescent coating is shown according to anadvantageous embodiment. The stress sensitive fluorescent coating canbe, for example, the stress sensitive fluorescent coating 614 of FIG. 6.Process 1000 begins at 1010 with the application of the stress sensitivefluorescent coating to a repaired component. The repaired component canbe a component or subassemblies produced in component and subassemblymanufacturing 106 processes shown in FIG. 1, including repairs made tothe component or subassemblies. The stress sensitive fluorescent coatingapplied at step 1010 may be, for example, the stress sensitivefluorescent coating 614 of FIG. 6. The stress sensitive fluorescentcoating includes the fluorescent dye molecules 616 whose fluorescentbehaviors change in response to external stress or deformation stimuli.The stress sensitive fluorescent coating may include a compatibleprimer, and a protective topcoat, or a layer of composite material or anappliqué installed over a repair. At step 1020, an initial stressprofile for the stress sensitive fluorescent coating is obtained usingsuitable equipment, such as the photoluminescent device 418 of FIG. 4.The stress profile essentially comprises a mapping of specificfluorescence of components to areas of that component exhibiting thefluorescence. At step 1030 the stress profile is stored in a dataprocessing system, such as the data processing system 620 of FIG. 6.

Attention is now directed to FIG. 11, which illustrates a flowchart of aprocess for determining stress inconsistencies, according to anadvantageous embodiment. The process 1100 begins at 1110 by obtaining acurrent stress profile for the stress sensitive fluorescent coating. Thecurrent stress profile can be obtained using a photoluminescent devicesuch as the photoluminescent device 418 of FIG. 4. The stress profileconstitutes a map of specific fluorescence of components to areas of arepaired component exhibiting the fluorescence. Based on the currentstress profile obtained in step 1110, a previously obtained stressprofile is retrieved at step 1120. The previous stress profile can be,for example, the initial stress profile obtained in step 1020 of FIG.10. The previous stress profile can be obtained from an attached dataprocessing system, such as the data processing system 620 of FIG. 6.Based on the previous stress profile retrieved at step 1120, stressdifferences between the current stress profile and the previous stressprofile are identified at step 1130. Although not shown in FIGS. 10 and11, a similar method may be employed to detect stress differencesindicative of inconsistencies when the stress sensitive fluorescent dyesare incorporated into the top layer (ply) of a repaired compositestructure or a repair patch 928, as previously discussed. It should benoted here that while the method described above in connection with FIG.11 relies on stress differences to indicate possible inconsistencies,such inconsistencies may be indicated directly by an obtained stressprofile, without the need for referencing a previously obtained“baseline” stress profile.

As previously discussed, changes in fluorescent wavelength emission,either toward monomer-like behavior or dimer-like behavior, or changesin emission intensity due to quenching or aggregation-induced emission,are all detectable. By comparing an initial stress profile of aircraftcoatings to subsequent stress profiles, stress changes due toinconsistencies in a repaired component can be determined. Based on anyidentified stress differences, repaired components having stressinconsistencies corresponding to stress abnormalities may be identifiedat step 1140. It should be noted here that it may be possible to tailorthe stress sensitive dyes to respond to preselected levels of stress,and to respond in various ways. For example, the dyes may be tailored toturn off, turn on or change color response when a preselected level ofstress is induced in the stress sensitive fluorescent coating.

FIG. 12 is a flowchart of a method for servicing a repaired componenthaving an inconsistent stress profile, according to another embodiment.An inconsistent stress profile is a stress profile that differs from aninitial stress profile, and therefore indicates the presence ofinconsistencies that may be invisible or barely visible to a visualinspection. The method comprises a process 1200 that begins at step 1210by identifying stress inconsistencies in a repaired component. Thestress inconsistencies can be identified using a process such as process1100 shown in FIG. 11. Based on the stress inconsistency identified instep 1210, any components affected by the stress inconsistencyidentified in step 1210 are serviced at step 1220. This service mayinclude, for example, application of a scarf or a composite patch 928 tothe affected component, similar to the scarf 940 shown in FIG. 9. Atstep 1230, a stress sensitive fluorescent coating is applied to thecomponent serviced in step 1220. The stress sensitive fluorescentcoating can be, for example, the stress sensitive fluorescent coating614 of FIG. 6. In the case of a repair patch 928, the stress sensitivefluorescent coating 614 may be incorporated into a composite plyoverlying the repair patch 928, or into an appliqué placed over therepair patch 928. The stress sensitive fluorescent coating includes thefluorescent dye molecules 616 whose fluorescent behaviors change inresponse to external stress or deformation stimuli, such as that causedby an impact event, or disbonding, delamination or degradation of acomposite repair patch 928. The stress sensitive fluorescent coating caninclude a compatible primer, and a protective topcoat.

Next, at step 1240, an initial stress profile is obtained for the stresssensitive fluorescent coating of the serviced component. The initialstress profile can be obtained using a photoluminescent device such asthe photoluminescent device 418 of FIG. 4. The stress profile, whichcomprises a map of specific fluorescence of the serviced areas, isstored at step 1250 in a data processing system, such as the dataprocessing system 620 of FIG. 6.

Example 1

A coating system of an epoxy based primer and a polyurethane (PU)topcoat was prepared on a 0.1 millimeter polyethylene terephthalatesubstrate. DesoPrime 7501, available from PPG Aerospace, Pittsburgh,Pa., was selected as the epoxy-based primer. DesoPrime 7501 comprises acuring agent and epoxy monomers. The curing agent is a mixture of paintsolids, n-butyl alcohol, and aliphatic amines. The epoxy monomercomprises bisphenyl A and Epichlorohydrin-based resin in an acetonesolvent.

A modified stilbene-type fluorescent dye was synthesized and prepared asa dry powder. The modified stilbene-type molecules were prepared havingtert-butyl dimethyl silane end groups. The selected end groups wereselected to be non-reactive with other components of the epoxy-basedprimer. The modified stilbene-type fluorescent dye was added to theepoxy monomer in an amount of 8.2*10^-4 mol/L of epoxy monomer solution,which was measured using a conventional fluorescence probe (probe 6shown in FIG. 13).

The epoxy-based primer was prepared in a 1:1 mix ratio, by volume, ofcuring agent to epoxy monomers. The epoxy-based primer was then appliedto the polyethylene terephthalate substrate at a thickness of 20-30micrometers. The epoxy-based primer was then allowed to cure at roomtemperature over a period of 48 hours.

DESOTHANE8800, available from PPG Aerospace, Pittsburgh, Pa., wasselected as the polyurethane topcoat. DESOTHANE 8800 comprises a basecomponent, and activator component, and a thinner component. The basecomponent comprises 2-oxypanone, polymer with2,2-bis(hydroxymethyl)-1,3-propanediol, methyl amyl ketone, and styreneacrylic polymer. The activator comprises a homopolymer of hexamethylenediisocyanate. The thinner component comprises methyl amyl ketone, andethyl acetate.

The polyurethane topcoat was prepared in a 2:1:1 mix ratio, by volume,of base component to activator component, to thinner. The polyurethanetopcoat was then applied to the epoxy-based primer at a thickness of50-75 micrometers. The polyurethane topcoat was then allowed to cure atroom temperature over a period of 48 hours.

Example 2

A coating system of an epoxy based primer and a polyurethane topcoat wasprepared on a 0.1 millimeter polyethylene terephthalate substrate.DESOPRIME 7501, available from PPG Aerospace, Pittsburgh, Pa., wasselected as the epoxy-based primer. DESOPRIME 7501 comprises a curingagent and epoxy monomers. The curing agent is a mixture of paint solids,n-butyl alcohol, and aliphatic amines. The epoxy monomer comprisesbisphenyl A and Epichlorohydrin-based resin in an acetone solvent.

A modified stilbene-type fluorescent dye was synthesized and prepared asa dry powder. The modified stilbene-type molecules were prepared havinghydroxyl end groups. The selected end groups were selected to bereactive with other components of the epoxy-based primer, and becomepart of the thermoset network formed as the epoxy cures. The modifiedstilbene-type fluorescent dye was added to the epoxy monomer in anamount of 1.28*10^-3 mol/L of epoxy monomer solution, which was measuredusing a conventional fluorescence probe (probe 7 shown in FIG. 13).

The epoxy-based primer was prepared in a 1:1 mix ratio, by volume, ofcuring agent to epoxy monomers. The epoxy-based primer was then appliedto the polyethylene terephthalate substrate at a thickness of 20-30micrometers. The epoxy-based primer was then allowed to cure at roomtemperature over a period of 48 hours.

DESOTHANE 8800, available from PPG Aerospace, Pittsburgh, Pa., wasselected as the polyurethane topcoat. DESOTHANE 8800 comprises a basecomponent, and activator component, and a thinner component. The basecomponent comprises 2-oxypanone, polymer with2,2-bis(hydroxymethyl)-1,3-propanediol, methyl amyl ketone, and styreneacrylic polymer. The activator comprises a homopolymer of hexamethylenediisocyanate. The thinner component comprises methyl amyl ketone, andethyl acetate.

The polyurethane topcoat was prepared in a 2:1:1 mix ratio, by volume,of base component to activator component, to thinner. The polyurethanetopcoat was then applied to the epoxy-based primer at a thickness of50-75 micrometers. The polyurethane topcoat was then allowed to cure atroom temperature over a period of 48 hours.

After mixing the polyurethane and epoxy coatings, liquid samples weremeasured for cure characteristics in Differential Scanning calorimetry(DSC) using a Netzsch DSC-200 with a Netzsch TASC 414/3 controller(Netzsch Instruments, Burlington, Mass.). Samples were heated inaluminum DSC crucibles at 2° C. per minute from 30° C. to 200° C. Curedsolid epoxy and polyurethane samples were also tested using the sameprogram to measure any residual or incomplete cure behavior.

Cured epoxy films, both with and without dyes, exhibit none of thesebehaviors, showing smooth curves with no exothermic or endothermicevents. Therefore, the present example does not interfere with thecompletion of cure or solvent evaporation in the epoxy primer coating atthese concentrations.

Example 3

A coating system of an epoxy based primer and a polyurethane topcoat wasprepared on a 0.1 millimeter polyethylene terephthalate substrate.DESOPRIME 7501, available from PPG Aerospace, Pittsburgh, Pa., wasselected as the epoxy-based primer. DESOPRIME 7501 comprises a curingagent and epoxy monomers. The curing agent is a mixture of paint solids,n-butyl alcohol, and aliphatic amines. The epoxy monomer comprisesbisphenyl A and Epichlorohydrin-based resin in an acetone solvent.

The epoxy-based primer was prepared in a 1:1 mix ratio, by volume, ofcuring agent to epoxy monomers. The epoxy-based primer was then appliedto the polyethylene terephthalate substrate at a thickness of 20-30micrometers. The epoxy-based primer was then allowed to cure at roomtemperature over a period of 48 hours.

DESOTHANE 8800, available from PPG Aerospace, Pittsburgh, Pa., wasselected as the polyurethane topcoat. DESOTHANE 8800 comprises a basecomponent, and activator component, and a thinner component. The basecomponent comprises 2-oxypanone, polymer with2,2-bis(hydroxymethyl)-1,3-propanediol, methyl amyl ketone, and styreneacrylic polymer. The activator comprises a homopolymer of hexamethylenediisocyanate. The thinner component comprises methyl amyl ketone, andethyl acetate.

A modified stilbene-type fluorescent dye was synthesized and prepared asa dry powder. The modified stilbene-type molecules were prepared havingtert-butyl dimethyl silane end groups. The selected end groups wereselected to be non-reactive with other components of the polyurethanetopcoat. The modified stilbene-type fluorescent dye was added to thethinner component in an amount of 6.3*10^-4 mol/L of the thinnercomponent, which was measured using a conventional fluorescence probe(probe 6 shown in FIG. 14).

The polyurethane topcoat was prepared in a 2:1:1 mix ratio, by volume,of base component to activator component, to thinner. The polyurethanetopcoat was then applied to the epoxy-based primer at a thickness of50-75 micrometers. The polyurethane topcoat was then allowed to cure atroom temperature over a period of 48 hours.

Example 4

A coating system of an epoxy based primer and a polyurethane topcoat wasprepared on a 0.1 millimeter polyethylene terephthalate substrate.DESOPRIME 7501, available from PPG Aerospace, Pittsburgh, Pa., wasselected as the epoxy-based primer. DESOPRIME 7501 comprises a curingagent and epoxy monomers. The curing agent is a mixture of paint solids,n-butyl alcohol, and aliphatic amines. The epoxy monomer comprisesbisphenyl A and Epichlorohydrin-based resin in an acetone solvent.

The epoxy-based primer was prepared in a 1:1 mix ratio, by volume, ofcuring agent to epoxy monomers. The epoxy-based primer was then appliedto the polyethylene terephthalate substrate at a thickness of 20-30micrometers. The epoxy-based primer was then allowed to cure at roomtemperature over a period of 48 hours.

DESOTHANE 8800, available from PPG Aerospace, Pittsburgh, Pa., wasselected as the polyurethane topcoat. DESOTHANE 8800 comprises a basecomponent, and activator component, and a thinner component. The basecomponent comprises 2-oxypanone, polymer with2,2-bis(hydroxymethyl)-1,3-propanediol, methyl amyl ketone, and styreneacrylic polymer. The activator comprises a homopolymer of hexamethylenediisocyanate. The thinner component comprises methyl amyl ketone, andethyl acetate.

A modified stilbene-type fluorescent dyes were synthesized and preparedas a dry powder. The modified stilbene-type molecules were preparedhaving hydroxyl end groups. The selected end groups were selected to bereactive with other components of the polyurethane topcoat.Specifically, the hydroxyl end groups react with the isocyanate group ofthe pre-polyurethane monomers and become incorporated into thepolyurethane chain. The modified stilbene-type fluorescent dye was addedto the thinner component in an amount of 7.4*10^-4 mol/L of the thinnercomponent, which was measured using a conventional fluorescence probe(probe 7 shown in FIG. 14).

The polyurethane topcoat was prepared in a 2:1:1 mix ratio, by volume,of base component to activator component, to thinner. The polyurethanetopcoat was then applied to the epoxy-based primer at a thickness of50-75 micrometers. The polyurethane topcoat was then allowed to cure atroom temperature over a period of 48 hours.

Cure Characteristics

After mixing the polyurethane and epoxy coatings, liquid samples weremeasured for cure characteristics in Differential Scanning calorimetry(DSC) using a Netzsch DSC-200 with a Netzsch TASC 414/3 controller(Netzsch Instruments, Burlington, Mass.). Samples were heated inaluminum DSC crucibles at 2 C per minute from 30 C to 200 C. Cured solidepoxy and polyurethane samples were also tested using the same programto measure any residual or incomplete cure behavior.

FIG. 13 shows the Differential Scanning calorimetry scans resulting frommeasurements of uncured and cured epoxy films prepared in Example 1 andExample 2.

FIG. 14 shows the Differential Scanning calorimetry scans resulting frommeasurements of uncured and cured polyurethane coatings prepared inExample 3 and Example 4.

Cured polyurethane films, both with and without dyes, do not exhibiteither exothermic or endothermic behavior. Just as in epoxy coatings,this is taken as evidence that the dye molecules at these concentrationsdo not unduly hinder the polymerization reaction or the solventevaporation.

Glass Transition Temperatures

Glass transition temperatures of the coatings were determined usingDynamic Mechanical Analysis (DMA) performed in a PerkinElmer DMA 7einstrument (PerkinElmer Life and Analytical Sciences, Inc., Waltham,Mass.). Cured samples of coatings 1.-.3 mm in thickness were removedfrom the PET film. Samples were tested in the DMA for glass transitionin 3-point bend configuration with a 10 mm span length. Temperaturescans were performed from −50 C to 50 C. 5 samples of each coating anddye combination were tested, and statistical analysis was performedusing Student's T-test.

FIG. 15 shows representative plots of the Differential Scanningcalorimetry scans taken for epoxy and polyurethane coatings within theincorporated stilbene-type fluorescent dyes.

FIG. 16 shows Glass transition temperature measurements of epoxy andpolyurethane coatings with and without the incorporated stilbene-typefluorescent dyes. While the dyes appear to cause a slight increase inT_(g), the variances of the sample sets do not allow that conclusion tobe drawn. P-values resulting from a 2-tailed Student's T-test comparingT_(g) values are shown in the plot. Student's T-test requires p-valuesto be below at least 0.05 to conclude that two distributions came fromdifferent sample sets. Accordingly, the presence of stilbene-typefluorescent dyes in the epoxy and polyurethane coatings at the shownconcentrations did not affect the glass transition of the coatings.

Absorbance Spectra

Photoluminescent quantum yield (PLQY) and fluorescence emission spectrawere collected using a Hamamatsu Absolute PL Quantum Yield MeasurementSystem available from Hamamatsu K.K. Quantum yield values and PLemission spectra were measured using a fiber optic LED illuminationsource in an integrating sphere. The illumination wavelength chosen wasthe maximum absorbance wavelength of the solid polyurethane films, λ=498nm. Each sample was exposed for 44 μs, and results were averaged 200times. Samples from various locations within the gage length of thetensile specimen were tested. The quantum yield values and peak emissionwavelengths were averaged for each tensile specimen.

FIG. 17 shows the absorbance spectra for the tert-butyl dimethyl silanefunctionalized stilbene dyes at various concentrations in the “thinner”precursor of the polyurethane coating.

FIG. 18 shows the absorbance spectra for the hydroxyl functionalizedstilbene dyes at various concentrations in the “thinner” precursor ofthe polyurethane coating.

The absorbance spectra of the dyes at various concentrations in liquiddioxin of the epoxy and the methyl amyl ketone, ethyl acetate of thepolyurethane was collected for a range including the visible, 250-1100nm. Spectra were normalized about the dimer absorbance peak wavelengthand offset to 0 A at 800 nm, well beyond the absorbance activity.

FIG. 19 shows the absorbance of the tert-butyl dimethyl silanefunctionalized stilbene dyes in solid polyurethane films on glasssubstrates.

The absorbance spectra of the dyes in solid polyurethane films on glasswere collected over the same range as in liquid solvents. Filmthicknesses were measured and spectra were scaled by the film thickness,and offset to 0 A at 800 nm. It was not possible to collect spectra fromdyes in the epoxy primer due to the large percentage of paint solids,which scattered or absorbed the incident illumination much too strongly.

Spectra were scaled for variations in film thickness. The hydroxylfunctionalized stilbene dyes shows similar absorbance data. The dimerabsorbance peak at λ=498 nm shows strongly in the solid coatings,indicating that dyes exist in the aggregate state within the solid. Themonomer peak at λ=395 nm is only weakly visible in the highestconcentration of dye. This is partially attributed to the strongabsorbance of the polyurethane film itself at wavelengths at or belowabout 400 nm.

Tensile Testing & Fluorescence Imaging

FIG. 20 shows typical stress-strain curves for the PET-epoxy andPET-polyurethane bilayers.

FIG. 21 shows images from a tensile stress test of hydroxylfunctionalized stilbene dyes in an epoxy film at 1.28×10⁻³ mol/L.

The fluorescent images of hydroxyl functionalized stilbene dyes in epoxyshow the fluorescence intensity increasing and also shifting wavelength,from reddish-orange to a more light orange color, a shift to lowerwavelength emissions. This is consistent with the molecular behavior ofthe stilbene dyes. When a large number P hydroxyl functionalizedstilbene molecules are cross-linked on one side to the coating polymernetworks, tensile stress can cause a reduction in aggregation of thestilbene molecules. This reduction increases the relative monomerabsorption and emission, resulting in an overall lower wavelength ofemitted fluorescence.

FIG. 22 shows images from a tensile stress test of hydroxylfunctionalized stilbene dyes in a polyurethane film at 7.4×10⁻⁴ mol/L.

The images of hydroxyl functionalized stilbene dyes in polyurethane emitmore intensely at the highest level of strain than hydroxylfunctionalized stilbene dyes in epoxy. This more intense emission isconsistent with a shift to higher energy monomeric absorption andemission.

As discussed above, the advantageous embodiments disclosed hereinprovide an optical witness in the form of stress sensitive fluorescentdyes that are incorporated into a structural repair, and which allow theperformance of the repair to be quickly and easily monitored. The stresssensitive fluorescent dyes may be incorporated into the pigment of anappliqué placed over the repair area or into the resin of the surface oroverlay ply of the repair. The optical behavior of the fluorescent dyeschange as a function of a stress in the repair, thereby providing avisual indication of the changes in the repair.

The description of the different advantageous embodiments has beenpresented for purposes of illustration and description, and is notintended to be exhaustive or limited to the embodiments in the formdisclosed. Many modifications and variations will be apparent to thoseof ordinary skill in the art.

For example, although an advantageous embodiment has been described withrespect to aircraft, the advantageous embodiment may be applied to othertypes of platforms that may have composite structures. For example,without limitation, other advantageous embodiments may be applied to amobile platform, a stationary platform, a land-based structure, anaquatic-based structure, a space-based structure and/or some othersuitable object. More specifically, the different advantageousembodiments may be applied to, for example, without limitation, asubmarine, a bus, a personnel carrier, tank, a train, an automobile, aspacecraft, a space station, a satellite, a surface ship, a power plant,a dam, a manufacturing facility, a building and/or some other suitableobject.

Further, different advantageous embodiments may provide differentadvantages as compared to other advantageous embodiments. The embodimentor embodiments selected are chosen and described in order to bestexplain the principles of the embodiments, the practical application,and to enable others of ordinary skill in the art to understand thedisclosure for various embodiments with various modifications as aresuited to the particular use contemplated.

What is claimed is:
 1. A structural repair, that comprises: a patchadapted to be adhesively bonded to a structure that requires a repair;and a dye, incorporated directly into a resin within a polymer networkin the patch, configured to indicate a current stress and load carryingability of the patch, such that the dye comprises: a sensitivity to bothtensile and compressive stresses; an end group that controls acombination of the dye with the polymer network; and an aggregationsensitivity that comprises fluorescent molecules that, responsive to achange in an aggregation behavior of the dye, visually indicates atleast one of partial failure or degradation in the repair such that eachfluorescent molecule, in the fluorescent molecules, respectivelycomprises the end group configured to control an electron density of thefluorescent molecules and reactivity with components of the polymernetwork.
 2. The structural repair of claim 1, wherein: the dye comprisesa mechanochromatic dye that comprises an electromagnetic response. 3.The structural repair of claim 2, further comprising the patchconfigured to bond to the structure, such that the fluorescent moleculescomprise a molecular mobility configured to control, responsive todeformation of the structure, a proximity between dye molecules withinthe resin.
 4. The structural repair of claim 2, wherein themechanochromatic dye comprises the fluorescent molecules, which comprisea fluorescence and positions in the resin, the fluorescent moleculesconfigured via selection of their respective end group, such thatresponsive to a change in a localized stress in the repair, a change inmolecules of the fluorescent molecules comprises a change in thefluorescence of the fluorescent molecule, such that responsive toelectromagnetic energy on the patch, the change in the fluorescencebecomes visible.
 5. A system that monitors changes in a patch bonded toa structure, such that the system comprises: a patch that comprises anumber of fiber reinforced resin plies; a resin ply cocured within thepatch and configured to indicate a current stress and load carryingability of the patch, such that the patch comprises a polymer networkconfigured to attach over a portion of skin of the structure, such thatthe resin ply comprises: fiber reinforced resin; and an electromagneticresponsive dye incorporated directly into the resin that comprises: asensitivity to both tensile and compressive stresses; an end group thatcontrols a combination of the electromagnetic responsive dye with thepolymer network; and an aggregation sensitivity that comprises afluorescent molecule that comprises: a molecular mobility of thefluorescent molecule relative to the polymer network in the patchconfigured to control, responsive to deformation of the structure, aproximity between molecules of the electromagnetic responsive dye; theend group configured to control, responsive to changes in stress in thepatch, an electron density and reactivity, with components of thepolymer network, of the fluorescent molecule; and a device configuredto: record a baseline image that represents an optical behavior of theresin ply at a time that the resin ply and the patch are cocured, suchthat the baseline image comprises a first stress profile for the patchas indicated by the resin ply; and produce an image that represents,responsive to a change in at least one of: a quenching, and anaggregation induced emission from the electromagnetic responsive dye, anew optical behavior of the resin ply that represents a new stressprofile in the resin ply, such that the new stress profile in the resinply comprises arises from at least one of: a delamination in the numberof fiber reinforced resin plies; and a disbanding of the patch; andsubject the resin ply to electromagnetic energy of a preselectedwavelength; record the optical behavior of the resin ply; collectphotoluminescent quantum yield and fluorescence emission spectra fromthe resin ply; compare the first stress profile and the new stressprofile.
 6. The system of claim 5, further comprising the devicecomprising a photoluminescent device.
 7. The system of claim 5, furthercomprising the device configured to: mark, responsive to the opticalbehavior comprising an indication that indicates a change, responsive toa delamination between the number of fiber reinforced resin plies of thepatch, in the stress in the patch, an area of the structure containingthe patch; and perform, based upon a traditional nondestructiveevaluation technique that comprises one of: bond testing, and ultrasonicinspection, further non-destructive evaluation of the patch.
 8. A systemconfigured to repair an area of an aircraft component, such that thesystem comprises: a patch configured to bond to the aircraft componentand occupy a portion of the aircraft component over the area, such thatthe patch comprises a polymer network and a number of composite pliesthat comprise fiber reinforced resin; a composite ply, in the number ofcomposite plies, that comprises: an electromagnetic responsive dyeincorporated directly into the fiber reinforced resin configured toindicate a current stress and a current load carrying ability in thepatch, such that the electromagnetic responsive dye comprises: asensitivity to both tensile and compressive stresses; an end group thatcontrols a combination of the electromagnetic responsive dye with thepolymer network; and an aggregation sensitivity that comprises afluorescent molecule that, responsive to a change in an aggregationbehavior of the electromagnetic responsive dye, presents an opticalbehavior that varies in response to changes in the current stress in thepatch, that comprises a customized end group configured to control anelectron density of the fluorescent molecule and reactivity withcomponents of the polymer network and the electromagnetic responsive dyecomprises a functionalized stilbene dye that comprises tert-butyldimethyl silane end groups; a photoluminescent device configured to:record a baseline image that represents the optical behavior of thepatch at a time that the patch bonds to the aircraft component, suchthat the baseline image comprises a first stress profile for the patch;record subsequent images of the patch that represent the opticalbehavior of the patch; obtain a new image of the patch, such that thenew image represents the optical behavior of the patch and comprises anew stress profile, based upon a delamination between the number ofcomposite plies, for the patch; compare the new image to the baselineimage and the first stress profile to the new stress profile; andidentify any abnormalities in the new stress profile, such that thefirst stress profile and the new stress profile each comprises a mappingof a fluorescence, unique to the respective stress profile of the patch,to areas that exhibit the fluorescence.
 9. The system of claim 8,further comprising the photoluminescent device configured to record eachnew image via being configured to: subject the patch to electromagneticenergy of a preselected wavelength; and collect photoluminescent quantumyield and fluorescence emission spectra from the patch.
 10. The systemof claim 8, further comprising the photoluminescent device configuredto: mark an area of the aircraft component containing the patch whencomparison of the new image to the baseline image indicates a change inthe current stress in the patch, and perform further non-destructiveevaluation of the patch using a traditional nondestructive evaluationtechnique comprising one of bond testing and ultrasonic inspection. 11.The system of claim 8, further comprising a data processing systemconfigured to recognize at least one of partial failure or degradationin the patch based upon distinctions between the new image and thebaseline image.